This invention relates to reducing defects caused during electrographic printing, and more particularly to reducing erroneous background writing in an electrographic printing environment.
Electrographic marking, or printing, upon an image recording medium comprises a two-stage process. First, ions are created by electrical breakdown of air in a gap between writing nibs and a recording medium, and ions (usually negative) are conducted to selected image pixel locations to form an electrostatic image on the recording medium. Second, the electrostatic image is made visible by "toning", which usually involves the passing of the recording medium, bearing the non-visible, electrostatic image, into contact with a liquid solution containing positively charged dye particles in colloidal suspension. The dye particles are attracted to the negative charge pattern and the density of the dyed image will be proportional to the potential or charge density on the medium.
Two types of recording media in common usage are paper and film. For paper, the bulk is treated to make it conductive and a dielectric layer of about 0.5 mil thick is coated upon its image bearing side. For film media, a substrate such as Mylar.RTM., is given a very thin conductive layer and an overcoat dielectric layer upon its image bearing side.
In the electrographic printing process, electrical contact must be made to the conductive layer of the medium in order to charge the dielectric layer with the electrostatic image. In the case of paper, this can be accomplished by direct electrical contact with the backplates of the writing device to the "backside" of the base paper. In the case of film, conductive edge stripes pass through the dielectric layer to the conductive layer providing electrical paths to the conductive layer. Electrical contact is made to the conductive layer through these stripes.
In the process, there must also be a means for establishing the electrical potential difference between the conductive layer and the nibs appropriate for electrical breakdown of the air in the gap. In one type of electrographic writing on paper, the potential of the conductive base is established by pulsing a backplate, which has resistive/capacitive coupling, to the back of the medium (across the medium) in conjunction with activation of writing nibs. The pulsing of a particular backplate in this case addresses several nibs in a group, and the pulsing is to activate that nib group. Pulsing of the backplate is used in what is known in the art as the multiplexed writing method where, during the writing state, the voltage applied to the nib could be -300 volts and the voltage applied to the backplate corresponding to the group including that nib could be pulsed +400 volts therefore providing 700 volts for writing. In the multiplexed writing method, groups of nibs are activated by a common set of drivers, but only those in conjuction with a complementary backplate will write, therefore requiring fewer drivers.
Similarly, the case of using the multiplexed writing method with film, the potential of the conductive layer is also established by pulsing a backplate in conjunction with the activation of the writing nibs, which in this case is only capacitively coupled to the conductive layer through the Mylar.RTM. base. The latter is shown in U.S. Pat. No. 4,424,522 to Lloyd et al., assigned to the same assignee as this application, and hereby incorporated by reference, and further describes multiplexed writing.
Another method of electrographic writing is the non-multiplexed electrographic writing method. In this case, each nib has its own driver and must be addressed individually. When using the non-multiplexed writing method, the potential of the conductive layer is established by grounding or DC biasing the back side of the paper, or the stripe on the film in conjuction with the activation of the nibs. No electrically active backplate is needed. The voltage applied to the conductive layer of the medium via the backplate is the same for all nibs unlike in the multiplexed case where the pulsing of an individual backplate addresses a group of nibs. In the non-multiplexing case, a low voltage such as -225 volts is applied to the nibs while in the OFF state while the blackplate remains at a constant zero volts and no writing is taking place. The voltage on the nibs is then increased when writing is to occur. During writing, the voltage applied to the nib could increase to -700 volts while the voltage applied on the blackplate remains at zero volts again providing 700 volts for writing. As will become apparent, the subject of the present invention can be used with both the non-multiplexed type writing method, where the backplate is held at a constant voltage, and the multiplexed writing method, where a specific backplate addressing a group of nibs is pulsed during writing.
For background purposes, referring to FIG. 1, shown is a model for explanation of the phenomenon occurring in the charging process via electrographic head 20 and recording medium 30. For clarity, only one nib 24 is shown but it can be appreciated that many nibs, positioned in a longitudinally extending array or nibline, are housed in head 20. Nib 24 is formed on substrate 22 and is connected to lead line 23 for supplying charging voltage to nib 24 for writing activation. Air in gap 27 exists between the end of nib 24 and the surface of recording medium 30 in order that the medium surface may be charged, or receive deposited charge. Medium 30 comprises a dielectric layer 32 deposited on a conductive base 34.
During writing, a charging voltage is applied to nib 24 while the conductive layer 34 of medium 30 is held at a DC bias level. Because of the electric field concentrations during charging of nib 24, there is a field emission 29 of electrons at the edges of nib 24. These electrons cause an ionization of air in gap 27. This ionization ignites a glow discharge in discharge region 28, near the central portion of nib 24 surrounded by field emission 29. The portion of gap 27 represented by discharge region 28 becomes ionized and therefore conductive. Discharge region 28 charges up the medium to a voltage where the voltage across the air gap drops to the glow discharge maintenance voltage. When the voltage drop reaches the glow discharge maintenance voltage, discharge region 28 will be extinguished and the charge deposition on the surface of medium 30 will cease.
Typically in an electrographic writing system, when electrographic head 20 is not writing (i.e. in an OFF state), nib 24 is held at a constant voltage below the threshold writing voltage known as an OFF voltage or V.sub.OFF. This OFF voltage is usually a constant DC voltage which is chosen such that writing does not occur but at a high enough voltage such that future writing will occur within a short time interval. Both the switching speed needed and the amount of writing voltage needed necessitates that the OFF voltage be elevated in the direction of an ON writing voltage rather than at a level which minimizes the field in the air gap. As described above, those nibs then chosen for writing are given an additional voltage, above the OFF voltage, bringing the charge on the writing nibs up to the ON writing voltage level. This ON voltage level brings the nibs to a state guaranteeing air gap breakdown and electrostatic writing.
A problem with having an elevated OFF voltage is the presence of an air gap field which allows charge to accumulate on the recording medium. This excess charge causes the toner to deposit onto the surface of the recording medium, contaminating the image. U.S. Pat. No. 4,290,076 to McFarland, and assigned to a common assigned as this application, addresses this problem and offers a compensation circuit as a solution to remove the excess charge.
Another problem caused by an elevated OFF voltage is present which causes image quality defects. During operation, the print medium comes in direct contact with the nibs. The medium causes an abrasive attack on the nib surface creating metal filaments which act as field amplifiers. The amplified field can often cause charge transfer to occur resulting in random background writing. Use of a less abrasive medium often reduces the amount of background writing but a reduction in abrasiveness lessens the ability of the media to clean the nibs. Lack of the cleaning function for the nibs would cause a greater amount of dropout or image loss. Therefore, changing the abrasiveness of the print medium is not a desirable solution.
Therefore, it would be desirable to reduce the random background writing from the elevated OFF voltage while still maintaining an OFF voltage with a potential high enough to guarantee rapid writing when writing is warranted. Therefore, a system is needed in which the OFF voltage can be elevated to a necessary level while erroneous background writing is reduced without causing dropout or other image quality defects.